Sugars, such as sucrose, fructose and glucose, are utilized to provide a pleasant taste to beverages, foods, pharmaceuticals, oral hygienic and cosmetic products. Sucrose, in particular, imparts a taste preferred by most consumers. Although sucrose provides superior sweetness characteristics, it is caloric. Non-caloric or lower caloric sweeteners have been introduced to satisfy consumer demand. Consumers also desire that these sweeteners have favorable taste characteristics.
Stevia is a genus of about 240 species of herbs and shrubs in the sunflower family (Asteraceae), native to subtropical and tropical regions and from western North America to South America. The species Stevia rebaudiana, commonly known as sweetleaf, sweet leaf, sugarleaf, or simply stevia, is widely grown for its sweet leaves. Stevia-based sweeteners may be obtained by extracting one or more sweet compounds from the leaves. Many of these compounds are glycosides of steviol, a diterpene compound. These diterpene glycosides are about 150 to 450 times sweeter than sugar. Steviol glycosides differ from each other by sweetness power as well as other sensory features contributing to taste quality such as bitterness, lingering aftertaste and the like. See Kinghorn, A. D., Stevia: The genus Stevia, Taylor & Francis, London (2002).
Examples of steviol glycosides are described in PCT International Patent Application Publication No. WO 2013/096420 (see, e.g., listing in FIG. 1); and in Ohta et. al., “Characterization of Novel Steviol Glycosides from Leaves of Stevia rebaudiana Morita,” J. Appl. Glycosi., 57, 199-209 (2010) (see, e.g., Table 4 at p. 204). Structurally, the diterpene glycosides are characterized by a single base, steviol, and differ by the presence of carbohydrate residues at positions C13 and C19, as presented in FIGS. 2a-2k of WO 20013/096420.
Typically, on a dry weight basis, the four major steviol glycosides found in the leaves of Stevia are dulcoside A (0.3%), rebaudioside C (0.6-1.0%), rebaudioside A (3.8%) and stevioside (9.1%). Other glycosides identified in Stevia extract include one or more of rebaudioside B, D, E, F, G, H, I, J, K, L, M, N, O, steviolbioside and rubusoside.
While the major steviol glycoside, rebaudioside A, is commonly used as sweetener in beverage applications it has off-taste issues. More recently, there has been focus on certain minor steviol glycosides which have better taste properties. For example, rebaudioside M has higher sweetness intensity and is more potent than other steviol glycosides (e.g., see Prakash, L, et al. (2013) Nat. Prod. Commun., 8: 1523-1526, and WO 2013/096420). Rebaudioside D tastes about 200-220 times sweeter than sucrose and in a sensory evaluation had a slow onset of sweetness and was very clean, namely sweeter overall than sucrose, less sweet lingering
aftertaste compared to sucrose (e.g., see Prakash, I., et al. (2012) Int. J. Mol. Sci., 13:15126-15136).
Recombinant DNA technology has made it possible, and commercially viable, to produce desired steviol glycosides using a wide variety of host cells such as yeast. Recovery of intracellularly expressed products from within the host cells, however, is fraught with difficulties because of the necessity of rupturing or hydrolyzing the tough yeast cell wall. The choice of cell wall disruption can also affect the downstream processing of the recovered product and its subsequent purification from yeast cells or fermentation broths thereby affecting the efficiency and cost effectiveness of the process.